This invention relates to a demand system for use with pneumatic sources and more particularly an electronic demand system for controlling the airflow from a pneumatic source linearly in response to changes in airflow demand.
In order to compete successfully in the airline industry, airline operators must minimize the expenses and departure delays included in maintaining and servicing its equipment. These factors have a major impact on operators because jet aircraft require virtually constant attention to operate reliably and safely. Thus, in order to survive in the marketplace, aircraft operators must depend upon reliable and cost effective aircraft servicing units whenever possible.
An example of the servicing regularly required for jet aircraft includes start-up of the jet engines. Pneumatic sources, including systems commonly referred to as air start units, conveniently couple to the aircraft and provide pneumatic power for actuating a pneumatic start system built into the jet engine which, in turn, drives and starts the engine. Air start units also typically double as auxiliary sources of air conditioning power for the aircraft while on the ground.
Because of stringent safety requirements for jet aircraft, air start systems must have the capability of providing variable airflow to the aircraft through a service hose connected to a discharge manifold at a substantially constant pressure. This generally requires a pneumatic control system for reliable pressure regulation of the air start unit. Damage to the aircraft can occur if the pressure is not sufficiently well regulated.
One conventional configuration for an air start unit having a pneumatic control system includes a compressor driven by a diesel engine and having an output directed into a discharge manifold. The compressor engine includes a mechanical governor for controlling its speed, and consequently, compressor output airflow. A bypass control valve is plumbed into the side of the discharge manifold to sense the discharge pressure for comparison with a regulated reference pressure. A differential in the discharge and reference pressures causes the bypass valve to either vent off excess pressure, or close to build up pressure within the manifold.
To assist the bypass valve in reducing or increasing pressure as needed in the manifold, the above unit incorporates a mechanical demand control system for varying the compressor engine speed. The system comprises first and second pneumatic actuators having respective pressure inputs communicating with the manifold and control pressure line respectively. Variations in the manifold pressure cause a proportional forward or rearward displacement of the first actuator to mechanically alter the engine throttle linkage. The second actuator is kept at a relatively constant level to maintain a minimum engine idle level.
Another conventional air start unit design incorporates a small pneumatic valve that attaches to the large bypass valve to control the throttle linkage of the engine. During operation, mechanical movement in the main bypass valve translates to direct mechanical movement of the small pneumatic valve via a mechanical linkage to either reduce or increase the pneumatic pressure to a single pneumatic actuator on the throttle.
While the conventional configurations described above work well for their intended purposes, several problems result from the use and operation of the associated demand systems. First and foremost are the system's inherent mechanical nature that necessitate precise adjustments and servicing to maintain satisfactory operation. Those skilled in the art have realized the complexity of such demand systems and often must invest substantial amounts of time and money in specialized mechanic training programs to maintain the units in operating condition. Another problem involves the relatively poor response characteristics of the demand systems that often cause the respective bypass valves to "hunt" for the proper level of closure to stabilize the manifold pressure. Lastly, mechanical demand systems that interface with mechanical governors are often incompatible for use with modern diesel engines incorporating electronic control governors to meet stringent emissions control regulations.
A further proposal to solve many of the aforementioned problems, marketed by the assignee of the present invention, includes many of the same features of the first proposal. However, instead of relying upon complex mechanical systems, the proposal instead incorporates a switch in the bypass valve to detect a threshold level of valve travel. The amount of valve travel indirectly indicates the presence of diverted airflow through the bypass. The switch output is fed back to an electronic governor in the compressor engine to step the engine output up or down according to predetermined set threshold levels. Because of the electrical characteristics of the feedback switch, the second proposal is easily implemented in modern diesel engine designs requiring electrical inputs for control of the electronic governor. However, the stepped threshold nature of the switch fails to adequately address the response problems typically associated with the mechanical based systems. Additionally, reliable operability of the switch often requires regular adjustments and specialized training.
Thus, those skilled in the art have recognized the need for an improved demand system for use with a pneumatic source to interface with modern diesel-powered compressors and provide reliable, straightforward, and relatively maintenance-free operation. The demand system of the present invention satisfies this need.